Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/02—Hardening by precipitation
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/04—Hardening by cooling below 0 degrees Celsius
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
  • C21D1/58—Oils

Definitions

• Hardened martensitic steel with low or no cobalt content process for producing a workpiece from this steel, and workpiece thus obtained.
• the invention relates to a martensitic steel hardened by a duplex system, that is to say by a precipitation of intermetallic compounds and carbides obtained by means of a suitable steel composition and heat aging treatment.
• This steel is said to be “duplex-hardening” because its hardening is achieved by simultaneous hardening precipitation of intermetallic compounds and M 2 C carbides.
• this steel still contains relatively large amounts of cobalt.
• this element is in any case expensive and its price is likely to undergo significant fluctuations on the raw materials market, it would be important to find ways to reduce its presence even more significantly, particularly in materials intended for more common mechanical applications. than aeronautical applications.
• the object of the invention is to provide a usable steel, in particular, for manufacturing mechanical parts such as transmission shafts, or structural elements, having a further improved mechanical resistance to heat but also fatigue properties and fragility. always adapted to these uses.
• This steel should also have a lower production cost than the best performing steels currently known for these uses, thanks, in particular, to a significantly lower cobalt content.
• the invention relates to a steel characterized in that its composition is, in percentages by weight:
• Ti + Zr / 2 traces - 100 ppm with Ti + Zr / 2 ⁇ 10 N
• It preferably contains C 0.20 - 0.25%.
• It preferably contains Cr 2 - 4%.
• It preferably contains Al 1 - 1, 6%, better 1, 4 - 1, 6%.
• It preferably contains Mo + W / 2 1 - 2%.
• It preferably contains V 0.2 - 0.3%.
• It preferably contains Ni 12-14%, with Ni> 7 + 3.5 Al.
• Nb traces - 0.05%.
• It preferably contains Si traces - 0.25%, better traces - 0.10%.
• It preferably contains O traces - 10 ppm.
• N traces - 10 ppm.
• It preferably contains S traces - 10 ppm, better traces - 5 ppm.
• It preferably contains P traces - 100 ppm.
• Its martensitic transformation temperature Ms measured is preferably greater than or equal to 100O.
• Its martensitic transformation temperature Ms measured may be greater than or equal to 140 ° C.
• the invention also relates to a method for manufacturing a steel part, characterized in that it comprises the following steps preceding the completion of the part giving it its final shape:
• curing aging at 475-600 ° C, preferably 490-525 ° C for 5-2 h. It further preferably comprises a cryogenic treatment at -50 ° C or lower, preferably at -80 ° C or lower, to convert all the austenite to martensite, the temperature being less than or equal to 150 ° C. Measured ms, at least one of said treatments lasting at least 4h and at most 50h.
• It further preferably comprises a softening treatment of the rough quenching martensite carried out at 150-250 ° C during 4-16h, followed by cooling with still air.
• the workpiece is preferably also carburized or nitrided or carbonitrided.
• Nitriding can be performed during an aging cycle.
• Said nitriding or carburising or carbonitriding can be carried out during a thermal cycle prior to or simultaneously with said dissolution.
• the invention also relates to a mechanical part or component for structural element, characterized in that it is manufactured according to the preceding method.
• It may be in particular a motor transmission shaft, or a motor suspension device or a landing gear element or a gearbox element or a bearing axis.
• the invention is based first of all on a steel composition which differs from the prior art represented by WO-A-2006/114499 in particular by a very low content of Co, not exceeding 1 %, and can be typically limited to the traces inevitably resulting from the elaboration.
• the contents of the other most commonly present significant alloying elements are only slightly modified, but certain levels of impurities must be carefully controlled.
• steels have a plastic gap (difference between breaking strength R m and resistance to elongation R p o, 2) intermediate between those of carbon steels and maragings. For the latter, the difference is very small, providing a high elastic limit, but a quick break as soon as it is crossed.
• the steels of the invention have, from this point of view, properties that can be adjusted by the proportion of hardening phases and / or carbon.
• the steel of the invention can be machined in the quenched state, with tools adapted to a hardness of 45HRC. It is intermediate between the maragings (rough processors of quenching since they have a mild low carbon martensite) and the carbon steels which must be machined essentially in the annealed state.
• a "duplex" curing is carried out, that is to say obtained jointly by intermetallics of ⁇ -NiAI type and by carbides of M 2 C type, in the presence of of reversion austenite formed / stabilized by diffusion-enriched nickel enrichment during curing aging, which gives ductility to the structure by forming a sandwich structure (a few% of stable and ductile austenite between the slats of hardened martensite).
• nitrides Ti, Zr and AI in particular, which are weakening: they deteriorate the tenacity and fatigue resistance. Since these nitrides can precipitate from 1 to a few ppm of N in the presence of Ti, Zr and / or Al, and the conventional elaboration means make it difficult to achieve less than 5 ppm of N, the steel of the invention respects the following rules.
• any addition of Ti (maximum allowed: 100 ppm) is limited, and N is limited as much as possible.
• the N content should not exceed 20 ppm and more preferably 10 ppm, and the Ti content should not exceed 10 times the N content. Nevertheless, a proportionate addition of titanium at the end of elaboration in a vacuum furnace is conceivable with a view to fixing the residual nitrogen and thus avoiding the harmful precipitation of the nitride AlN.
• the addition of titanium can be practiced only for a residual maximum nitrogen content of 10 ppm in the liquid metal, and still not exceed 10 times this residual value of nitrogen.
• the limiting content of the optional addition of titanium is 80 ppm.
• Ti and Zr are to be regarded as impurities to be avoided, and the sum Ti + Zr / 2 must not exceed 150 ppm.
• rare earths at the end of the process, can also help to fix a fraction of N, besides the S and O. In this case, it must be ensured that the residual rare earth content remains below 100. ppm, and preferably less than 50 ppm, because these elements weaken the steel when they are present beyond these values. It is believed that rare earth (eg La) oxynitrides are less harmful than Ti or Al nitrides because of their globular form which would make them less likely to constitute fatigue fracture primers. It is nevertheless advantageous to leave as few of these inclusions as possible in the steel, thanks to the classic techniques of careful elaboration.
• Calcium treatment may be practiced to complete the deoxidation / desulfurization of the liquid metal. This treatment is preferably conducted with the possible additions of Ti, Zr or rare earths.
• M 2 C carbide of Cr, Mo, W and V containing very little Fe is preferred for its hardening and non-embrittling properties.
• M 2 C carbide is tastable against equilibrium carbides M 7 C 3 and / or M 6 C and / or M 2 3C 6 . It is stabilized by Mo and W.
• Mo + W / 2 is between 1 and 2%. It is also to prevent the formation of non-hardening Ti carbides which may weaken the grain boundaries that a 100 ppm imperative limitation of the Ti content of the steels according to the invention is required.
• Cr and V are elements that activate the formation of "metastable" carbides.
• V also forms carbides of MC type, stable up to the dissolution temperatures, which "block" the grain boundaries and limit the magnification of grains during heat treatments at high temperatures.
• V 0.3% must not be exceeded in order not to fix too much C in carbides of V, during the dissolution cycle, to the detriment of the M 2 C carbide of Cr, Mo, W, V which is sought precipitation during the subsequent aging cycle.
• the V content is between 0.2 and 0.3%.
• the presence of C favors the appearance of M 2 C with respect to the ⁇ phase.
• an excessive content causes segregations, a lowering of Ms and causes difficulties in manufacturing on an industrial scale: sensitivity to the taps (superficial cracking during rapid cooling), difficult machinability of martensite too hard to l quenching condition ...
• Its content must be between 0.20 and 0.30%, preferably 0.20-0.25% so as not to give the part too hard a hardness which could require machining in the annealed state.
• the surface layer of the parts can be enriched in C by cementation, nitriding or carbonitriding if a very high surface hardness is required in the intended applications.
• Co delays the restoration of dislocations and, therefore, slows the mechanisms of hot survivability in martensite. It was thought that this made it possible to maintain high tensile strength at high temperature. But on the other hand, it was suspected that, since the Co promotes the formation of the aforementioned ⁇ phase which is the one that hardens the maraging steels of the prior art to Fe-Ni-Co-Mo, its massive presence contributed to reducing the quantity of Mo and / or W available to form M 2 C carbides which contribute to the hardening according to the mechanism that is to be promoted.
• the cobalt somewhat raises the ductile / brittle transition temperature, which is not favorable, particularly in compositions with low nickel contents, whereas, contrary to what could be found in cobalt does not clearly show the transformation point Ms of the compositions of the invention and therefore has no obvious interest either in this respect.
• Ni and Al are linked in the invention, where Ni must be> 7 + 3.5 Al. These are the two essential elements which participate in a good part of aging hardening, thanks to the precipitation of the nanometric-type intermetallic phase.
• B2 NiAI for example. It is this phase which confers a large part of the mechanical strength to hot, up to about 400 0 C.
• the nickel is also the element which reduces brittleness by cleavage because it lowers the ductile / brittle transition temperature of martensites. If Al is too high with respect to Ni, the martensitic matrix is too strongly depleted of nickel as a result of the precipitation of the NiAI hardening precipitate during aging.
• martensitic transformation start temperature Ms which, according to the invention, should preferably remain equal to or greater than 140 ° C. if no cryogenic cycle is used, and should preferably be equal to or greater than 100 ° C. if we practice a cryogenic cycle.
• the end-of-cooling temperature after quenching should be less than the actual Ms -150 ° C, preferably less than the actual Ms -200 ° C, so to ensure a full martensitic transformation of steel.
• this end-of-cooling temperature can be obtained as a result of a cryogenic treatment applied immediately following cooling to ambient temperature from the solution temperature.
• the cryogenic treatment can also be applied not from room temperature, but after isothermal quenching ending at a temperature slightly greater than Ms, preferably between Ms and Ms + 50 ° C.
• the overall cooling rate must be the highest.
• the Ms value of the steel is greater than or equal to 100O if a cryogenic cycle is applied, and greater than or equal to 140O in the absence of this cryogenic cycle.
• the duration of the cryogenic cycle is between 4 and 50 hours, preferably from 4 to 16 hours, and more preferably from 4 to 8 hours. It is possible to practice several cryogenic cycles, the essential being that at least one of them has the aforementioned characteristics.
• the elastic limit R p o, 2 is influenced in the same way as R m .
• the steels of the class of the invention prefer the presence of the hardening phases B2. , especially NiAI, to obtain a high mechanical strength when hot. Compliance with the conditions on Ni and Al that have been given ensures a sufficient potential content of reversion austenite to maintain ductility and toughness suitable for the intended applications. It is possible to add B, but not more than 30ppm not to degrade the properties of the steel.
• Nb to control the grain size during forging or other hot processing, at a content not exceeding 0.1%, preferably not exceeding 0.05% for avoid segregations that may be excessive.
• the steel according to the invention therefore accepts raw materials that can contain significant residual contents in Nb.
• a characteristic of the steels of the class of the invention is also the possibility of replacing at least a portion of Mo by W.
• W segregates less at solidification than Mo and provides an increase in mechanical strength when hot. It has the disadvantage of being expensive and we can optimize this cost by associating it with Mo.
• Mo + W / 2 must be between 1 and 4%, preferably between 1 and 2% . It is preferred to maintain a minimum content of 1% Mo to limit the cost of steel, especially as the high temperature withstand is not a priority objective of the steel of the invention.
• Cu can be up to 1%. It is likely to participate in the hardening with the help of its epsilon phase, and the presence of Ni makes it possible to limit its harmful effects, in particular the appearance of superficial cracks during the forging of the pieces, which one observes during additions of copper in steels not containing nickel. But its presence is not essential and it may be present only in the state of residual traces, resulting from the pollution of raw materials.
• Manganese is not a priori useful for obtaining the properties of steel, but it has no recognized adverse effect; in addition, its low vapor pressure at the temperatures of the liquid steel makes it difficult to control its concentration in vacuum and vacuum remelting: its content may vary depending on the radial and axial location in a remelted ingot. As it is often present in the raw materials, and for the reasons above, its content will preferably be at most 0.25%, and in any case limited to 2% at the most because of too great variations of its concentration in a same product will interfere with the repeatability of the properties.
• Silicon is known to have a hardening effect in solid solution of ferrite and, like cobalt, to decrease the solubility of some elements or certain phases in ferrite. Nevertheless, the steel of the invention requires a significant addition of cobalt, and the same is true of the addition of silicon, especially since, moreover, silicon generally promotes the precipitation of intermetallic phases. harmful in complex steels (Laves phase, silicides ...) - Its content will be limited to 1%, preferably less than 0.25% and more preferably less than 0.1%.
• S traces - 20ppm, preferably traces - 10ppm, better traces - 5ppm
• P traces - 200ppm, preferably -100ppm traces, better traces-50ppm.
• Ca can be used as a deoxidizer and as a sulfur sensor, finding it in the end ( ⁇ 20ppm).
• rare earth residues may ultimately remain ( ⁇ 100ppm) as a result of liquid metal refining treatment where they would have been used to capture O, S and / or N.
• the use of Ca and rare earths for these effects not being mandatory, these elements may be present only in the form of traces in the steels of the invention.
• the acceptable oxygen content is 50 ppm maximum, preferably 10 ppm maximum.
• the reference steel A corresponds to a steel according to US-A-5 393 388, thus having a high Co content.
• Reference steel B corresponds to a steel comparable to steel A, to which V was added without modifying the content of Co.
• the reference steel C corresponds to a steel according to WO-A-2006/114499, in particular in that, with respect to the steels A and B, its Al content has been increased and its Co content has been increased.
• the reference steel D has undergone a C addition of B.
• the reference steel E has undergone a Nb addition to C.
• the reference steel F differs from C mainly by the absence of a significant addition of V, compensated by a lower C content, and a higher purity of residual elements.
• the reference steel G is distinguished from F by a very low content of Co which would be in accordance with the invention, the presence of V at a level comparable to that of C, D and E, and a higher Ni content, but which, taken in isolation, would nonetheless conform to the invention. But its contents in Ti and N are slightly higher than the invention tolerates. Experience also shows that its measured temperature Ms is substantially too low compared to the requirements of the invention, the relatively high Ni content is not compensated by relatively low levels of Cr, Mo, Al and V.
• the steel H is in accordance with the invention in all respects, in particular its very low Co content and its high N and Ti purity. Also, its O content is very low. Finally, its measured temperature Ms is entirely in accordance with the invention.
• the samples were softened at a temperature of at least 600 ° C.
• this softening income was made at 650 ° C for 8 hours and followed by cooling in the air. Thanks to this, the raw products of thermomechanical transformations can undergo without particular problems the finishing operations (straightening, peeling, machining ...) giving the piece its final form.
• cryogenic treatment at 80 ° C. for 8 hours; Specifically for sample H, another cryogenic treatment was added at -120 ° C for 2 h;
• the reference samples C, D and E have a tensile strength that is much greater than that of the reference samples A and B.
• the elastic limit is at least of the same order of magnitude.
• the properties of ductility (necking and elongation at break), toughness and resilience are lowered, in the case of heat treatments described and applied.
• the desired resistance / toughness compromise can be adjusted by changing the aging conditions.
• Reference sample B shows that the mere addition of V to steel A gives only an improvement in certain properties, and in proportions that are often less important than in the case of steels with reduced or no Co content.
• Sample G shows that the large decrease, up to the total removal, of cobalt, can still allow to maintain a high tensile strength.
• the ductility properties are also improved.
• the elastic limit is, however, quite substantially deteriorated in the case of the sample G, in relation to a larger amount of austenite dispersed in the structure, due to the high Ni content of this sample. This contributes to an excessive lowering of the measured Ms which is not compensated by adjustments of the contents of the other elements.
• sample H which conforms in every respect to the composition according to the invention, and whose temperature Ms is sufficiently high, we obtain: - a tensile strength which remains high, and could be, if necessary, further improved by an increase in the C content which would promote hardening by quenching and formation of secondary carbides; a tensile strength of the order of 2300 MPa would thus be accessible for a C content of about 0.25%;
• N and Ti a little too high in the sample G compared to the requirements of the invention, and also its slightly higher oxygen content, also contribute in part to its poorer performance than that of the sample.
• H Another factor to consider for this sample G is an S content which is not particularly low, and which tends to degrade toughness if it is not offset by other characteristics that would be favorable to this property.
• this sample G has a fairly high Ni content (although remaining within the range of the invention), which lowers Ms and thus promotes the maintenance of a possibly too high residual austenite level. , even after the cryogenic treatment more particularly pushed (at -80 "C and then at -120" C) which was undergone by c and sample.
• the sample H according to the invention which has been cryogenically treated only at -80.degree. C., but which has a junctionally adjusted Ni content, has minimal impurity contents in all respects. and a sufficiently high temperature Ms, responds very well to the problems posed.
• an optimized heat treatment mode of the steel according to the invention for finally obtaining a part having the desired properties is, after forming the blank of the part and before the completion. giving the piece its final form:
• a cryogenic treatment at -50O or lower, preferably at -80 ° C or lower, to convert all the austenite to martensite, the temperature being lower by 150 ° C or more at Ms, preferably lower than At least about 200 ° C, at least one of said cryogenic processes for at least 4 hours and at most 50 hours, for compositions having, in particular, a relatively low Ni content which leads to a relatively high MS temperature, this cryogenic treatment is less useful.
• thermomechanical heat-forming treatments may be performed in addition to or in place of this forging, depending on the type of end product that is desired (stamped parts, bars, semi-finished products. ..) - There may be mentioned one or more rolling, stamping, stamping ... and a combination of several such treatments.
• the preferred applications of the steel according to the invention are the endurance parts for mechanics and structural elements, for which a tensile strength of between 2000 MPa and 2350 MPa or more must be cold, combined with values ductility and resilience at least equivalent to those of the best high-strength and hot (400 ° C) steels a tensile strength of the order of 1800 MPa, as well as optimal fatigue properties.
• the steel according to the invention also has the advantage of being cementable, nitrurable and carbonitrurable.
• the parts that use it can therefore be given high abrasion resistance without affecting its core properties. This is particularly advantageous in the intended applications that have been cited.
• Other surface treatments, such as mechanical treatments that limit the initiation of fatigue cracking from superficial defects, are conceivable. Shot peening is an example of such treatment.
• nitriding is carried out, this can be carried out during the aging cycle, preferably at a temperature of 490 to 525 ° C and for a period of time ranging from 5 to 100 hours, the longest ages causing progressive structural softening and, as a result, a progressive decrease in the maximum tensile strength.
• Another possibility is to carry out carburizing, nitriding or carbonitriding during a thermal cycle prior to or simultaneously with the dissolution, the steel substrate of the invention retaining in this case all its potential mechanical properties.

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